As regards a laser imager (an image processing apparatus) for a medical use, a requirement for its basic function to output the image density always stably is very strong because a diagnosis image is expressed in a density gradation.
Further, in a laser imager for medical use, there is provided what is called a calibration function because its image formation section is controlled so as to make a digital or video signal (specifying density signal) transmitted from a diagnosis apparatus or a photographing apparatus a constant density on a film.
However, although a constant density can be obtained immediately after the practice of calibration, with the processing of the films after the calibration, image density fluctuates due to various kinds of factor. In particular, density fluctuation tends to occur in a process using a thermal development device; for example, density fluctuations as described below can be considered.    (1) a fluctuation in the exposure system caused by the temperature rise in the machine (for example, the fluctuation of the AOM light quantity and the LD wavelength);    (2) a fluctuation of the thermal development characteristics such as the temperature rise in the thermal development cooling-conveyance section caused by the film processing;    (3) a fluctuation of the sensitivity characteristics of a film preserved in the machine;    (4) a fluctuation of the characteristics of the thermal development drum due to the attachment of fatty acid or the like caused by the film processing; and    (5) use of films having different thermal development characteristics.
Among the above-mentioned fluctuations, as regards the fluctuations such as (1), (2), and (5), if the apparatus used is specified, a characteristic fluctuation caused by use can be estimated on the basis of a previous experiment, and it can be cancelled by a feed-forward control (to be abbreviated to “FF” as occasion demands).
On the other hand, as regards the fluctuations such as (3) and (4), because they are difficult to foresee previously, sometimes it is used what is called a patch density method in which the finish density which has been subject to the overall influence including the influences of (3) and (4) is measured and a feedback (to be abbreviated to “FB” as occasion demands) correction is applied to the image formations for next print.
This patch density method is such one that a rectangular area of an order of 5 mm×10 mm on a specified position of a film is exposed to a predetermined light quantity, the finish density of this area is measured, and on the basis of the difference from the density to be expected (hereinafter referred to as “a density for comparison”), by the application of an FB correction to make the images on the next image formation have an optimum density, the exposure quantity and/or thermal development conditions are changed.
As one of the patch density methods, it is known a laser recording apparatus (an image processing apparatus) which measures the film density after thermal development by means of a transmission type sensor and feeds back the result to the laser light quantity (for example, the publication of the unexamined patent application S62-249138).
However, in this method, even though there is a fluctuation or a measurement error in the patch density measurement, a correction is necessarily applied to the prints each time; this is not desirable.
Further, in a thermal development process, generally speaking, processing is prohibited until the heating drum and the surrounding members come to have a specified temperature; however, the development characteristic changes from one immediately after the apparatus becomes ready to one after a plurality of prints have been produced, and the image density tends to become high because the temperature of the cooling-conveyance section rises with the processing.
It is known an image recording apparatus which, in order to prevent this, detects the temperature of the cooling-conveyance section and applies a correction to the exposure (exposing device) means (for example, the publication of the unexamined patent application 2000-284382); thus, even though there is a fluctuation or a measurement error in the patch density measurement, the influence can be cancelled.
However, also the apparatus described in the above-mentioned patent application does not notice the influence of the deterioration of the film with the processing of the films, and the influence of it still exists; therefore, it has been difficult to stabilize the image density.
However, the exposure system and the thermal development system contains factors of dispersion among apparatus, and films change with the processing of the films; therefore, there is a problem that the setting of the amount of this characteristic change and the density for comparison is easy to become erroneous. Accordingly, if the setting of the above-mentioned amount of the characteristic change and the density for comparison is done erroneously, a density lowering or a density rise is produced as the result, even though the process systems reproduce a density in accordance with the setting.